The biggest on-line marketplace in the world is Alibaba. They charge for verification, up to $40K if I recall correctly. The thing is that you can join for free as a vendor, or at least you could about ten years ago when I posted a profile as part of research for another client.

ProSavvy was quite successful and made a profit as a freelance work broker board. Membership was perhaps $1.5K/year or so. I used an acquaintance's membership and gave him 10% of my net. I think I earned maybe $15-20K on them in a year.